The present invention relates to a sprocket assembly which has variable gear ratios, and, in particular, relates to a sprocket assembly which does not require a derailleur to change gears.
Although the primary mode of ground transportation is the automobile, the bicycle continues to be a popular mode of transportation, particularly for recreational purposes. One of the reasons that the bicycle maintains its popularity is the availability of bicycles having multiple gear ratios, for example, "10-speed" and "15-speed" models. While in higher gears, these bicycles enable a rider to obtain relatively high speeds, which is advantageous while traversing flat terrain or going downhill. Moreover, by shifting into lower gears, the rider can climb hills with relatively little effort.
The most common means for providing multiple gear ratios on a bicycle is a "derailleur." As is well known in the art, a derailleur system has a plurality of rear sprockets of varying diameters mounted proximate to and in driving engagement with the rear wheel of the bicycle. One or more front sprockets are typically mounted in so as to be driven by the pedal cranks. The periphery of each sprocket is provided with a plurality of teeth, the number of which vary with the diameter of the sprocket. A chain link drive chain meshes with the teeth and transmits power from a selected one of the front sprockets to a selected one of the rear sprockets.
To shift gears, the drive chain is moved from one sprocket to an adjacent sprocket to change the gear ratio between the front sprockets and the rear sprockets. Thus, the mechanical advantage applied to cause rotation of the rear wheel by the pedals is varied. Typically, a derailleur system will have five or six rear sprockets and two or three front sprockets. For example, a 10-speed bicycle will have two front sprockets and five rear sprockets. Similarly, a 15-speed bicycle will have three front sprockets and five rear sprockets.
Although the derailleur system is very popular and is adequate in most cases to provide the ability to shift between the available gear ratios, there are a number of inadequacies associated with the derailleur system that are familiar to anyone who has used such a system. For example, the drive chain must be physically moved laterally to shift the chain from one sprocket to an adjacent sprocket. To do this, the chain is physically pushed to one side during the shifting process to cause it to slip off one sprocket and onto the adjacent sprocket. Thus, during the shifting process, the chain is not aligned between the front driving sprocket and the rear driving sprocket. This misalignment causes wear on both the chain and the sprockets during the shifting process.
Another problem with exemplary derailleur systems is that the chain is not aligned between the front and rear sprockets for all gear ratios. For example, with many derailleur systems, it is not advisable to operate with the derailleur chain passing from the leftmost front sprocket to the rightmost rear sprocket, or from the rightmost front sprocket to the leftmost rear sprocket. During these operations, the chain is exerting lateral forces against the edges of the sprocket teeth, and can cause excessive wear.
Furthermore, it is preferable, if not necessary, that the shifting process occur when no heavy loads are present on the driving mechanism. For example, if a shift is initiated while the rider is exerting large forces on the drive mechanism while pedaling uphill, the wear on the chain and the sprockets will be increased substantially and there may be some difficulty in completing the shifting process.
Finally, the derailleur system only operates effectively when the bicycle is moving. For example, when the bicycle is standing still, the gear ratio on a derailleur system cannot be changed. If such an attempt is made, the derailleur will not operate properly upon resuming pedaling. Thus, it is necessary for a rider coming to a stop to anticipate the gear ratio required to resume motion and to downshift prior to stopping.
Other systems have been devised for providing variable gear ratios on bicycles. For example, U.S. Pat. No. 593,285 to Van Eyck, issued Nov. 9, 1897, shows an early gear-changing device using a segmented sprocketwheel that can be enlarged or reduced in size by means of a device located within reach of a bicycle rider. U.S. Pat. No. 2,827,797 to Bell, et al., issued on July 22, 1954, and U.S. Pat. No. 3,798,989 to Hunt, issued on Mar. 26, 1974, show multiple-speed drive sprockets that can be varied from a circular configuration to an elliptical configuration. U.S. Pat. No. 3,800,613 to Clark, issued on Apr. 2, 1974, shows a variable speed sprocket having a plurality of smaller sprockets that can be moved radially outward to vary the effective diameter of the sprocket group. U.S. Pat. No. 4,260,386 to Frohardt, issued on Apr. 7, 1981, has an outer gear ring for engagement with a standard bicycle chain and having inner segments that can be adjusted to different configurations for driving the outer gear ring. U.S. Pat. No. 4,493,678 to Husted, issued on Jan. 15, 1985, has a plurality of sprocket segments that are movable inwardly and outwardly in a corresponding number of spiral-wavy-cams.
Although the above-listed patents appear to describe sprockets that maintain chain alignment at each of the gear ratios, none of the above-described patents appear to disclose a drive sprocket that remains substantially circular for each of the gear ratios. It is believed that substantial circularity is preferable in order to provide the smooth pedaling effect obtained when riding a typical bicycle having sprockets which remain circular. For example, the elliptical sprockets disclosed by the Bell and Hunt patents will provide a different effective gear ratio depending upon the angular positions of the pedals. Thus, the rider must apply varying effort as the pedals are rotated about the crank axis. This change in the gear ratio is frequently referred to as "chordal speed variation" and is caused by the deviation of the sprocket from a perfect circle. Even a conventional sprocket has a chordal speed variation of approximately 2 percent caused by the use of a linked chain rather than a smooth belt. For example, when engaged with a chain, a 27-tooth sprocket would effectively have 27 flat surfaces or chords forming its circumference, as opposed to a smooth, truly circular surface. The chordal speed variation of the Clark device will be substantial because of the significant difference in the distance of the chain from the axle of the wheel when the sprockets are tangential to the chain and when the sprockets are disposed at an angle to the chain.
Thus, a need exists for a drive sprocket that provides a variable gear ratio while maintaining substantial circularity of the driving sprocket.
Another drawback of the derailleur systems is that the shifting of gears is accomplished manually, by moving a lever. The lever typically actuates a cable, which in turn causes the derailleur to shift gears. Each time the gears are to be shifted, the rider must loosen his grip on the handlebar of the bicycle in order to manually actuate the lever. Thus, when travelling at high speeds or over irregular terrain, maintaining of directional control over the bicycle can be difficult while the gears are being changed.
Automatic transmissions have been developed which automatically change gears without requiring manual actuation. However, these previous devices, which operate mechanically, have been unsatisfactory due to loss of efficiency which results from the additional mechanical components that engage the drive chain and cause the rider to have to exert additional effort to operate the bicycle. Further, these prior devices cause gear changes upon sensing variation in the amount of torque applied to the drive chain by the rider. Since torque variations are common when the rider temporarily discontinues pedalling and is coasting, such as during turns or when going downhill, the prior transmissions will often cause unwanted gear changes during short lulls in the pedalling action of the rider. Thus, a need exists for an improved automatic transmission that does not require the rider to have to exert extra effort in order to overcome the loss of efficiency caused by mechanical automatic transmissions, and which shifts in accordance with the speed of the bicycle rather than the torque applied to the chain.